This invention relates to the production of paper, and in particular, to the determination of the basis weight of a moving or stationary substrate, such as a web of paper.
Presently, the basis weight of a web of paper is determined using a single source which emits Beta or Gamma radiation particles. The radiation passes through the paper web and is measured by a detector on the other side of the web of paper. As it passes through the web, the beam of radiation is attenuated. The attenuation is proportional to the density and thickness of the web. Thus, the signal received is proportional to the basis weight of the paper web.
The source that is used is generally quite small, only an inch or two in diameter. Consequently, the detector must be mobile to measure the full width of the paper web. Presently, the detector is mounted on an endless belt and travels to and fro across the width of the paper. Thus, at any one time, the information received is only determinative of the density of the paper at a single point. To determine the basis weight of the paper, thousands of measurements have to be taken as the device scans across the paper. Because the paper is moving past the measuring device quickly, i.e. up to 7,000 FPM for newsprint, the time spent by the device in measuring a specific part of the web, in the cross direction, is limited. Further, because only an average basis weight, over many thousands of feet of paper, can be determined, it is not possible to use the present scanners to investigate short time span variables which affect basis weight and therefore affect product quality.